Nuclear fuel storage system with integral shimming

ABSTRACT

A nuclear fuel storage system includes an outer canister and fuel basket positioned therein. The basket is formed by orthogonally arranged and interlocked slotted plates which collectively define exterior side surfaces of the basket and a grid array of open cells each configured to hold a fuel assembly. At least some slotted plates comprise cantilevered plate extensions protruding laterally beyond the side surfaces of the basket to define various shaped peripheral gaps between the basket and canister. The plate extensions are configured to engage the shell of the canister. Vertically elongated reinforcement members are inserted in the peripheral gaps and fixedly coupled to the basket. Reinforcement members may comprise elongated reinforcement plates and/or tubular shimming members which may be fixedly coupled to the slotted plate extensions. The reinforcement members structurally strengthen the fuel basket. The plate extensions further act as fins to enhance heat dissipation from the basket.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.62/945,587 filed Dec. 9, 2019, which is incorporated herein by referencein its entirety.

BACKGROUND

The present invention relates generally to systems and apparatuses forstoring high level radioactive waste such as used or spent nuclear fuel(SNF), and more particularly to an improved system comprising a nuclearfuel basket with integral shims.

In the operation of nuclear reactors, the nuclear energy source is inthe form of hollow Zircaloy tubes filled with enriched uranium,collectively arranged in multiple assemblages referred to as fuelassemblies. When the energy in the fuel assembly has been depleted to acertain predetermined level, the used or “spent” nuclear fuel (SNF)assemblies are removed from the nuclear reactor. The standard structureused to package used or spent fuel assemblies discharged from lightwater reactors for off-site shipment or on-site dry storage is known asthe fuel basket. The fuel basket is essentially an assemblage ofprismatic storage cells each of which is sized to store one fuelassembly that comprises a plurality of individual spent nuclear fuelrods. The fuel basket is arranged inside a cylindrical metallic fuelstorage canister, which is often referred to as a multi-purpose canister(MPC) that forms the primary nuclear waste containment barrier. SuchMPCs are available from Holtec International of Camden, N.J. The fuelassemblies are typically loaded into the canister while submerged in thespent fuel pool of the reactor containment structure to minimizeradiation exposure to personnel.

The typical prismatic structure of the fuel basket used to store the SNFcomprises openings or cells; each of which houses a single fuel assemblyas previously noted. The cells have a square cross-sectional shape tomatch the configuration of U.S. type PWR (Pressurized Water Reactor)fuel assemblies. The fuel basket walls may be made by stacking andmechanically interlocking slotted plates together which are arranged inan orthogonal pattern to achieve the desired height. All of the exteriorplates are typically welded together to structurally stabilize thestack. The resultant cells may have a square cross-sectional shapes tomatch the configuration of U.S. type PWR (Pressurized Water Reactor)fuel assemblies. FIGS. 1 and 2A show a typical fuel basket structurehaving a plurality of undulating planar stepped side walls surfacesformed by the exterior outboard slotted plates. The basket has acompound rectilinear polygonal prismatic perimeter in top down view;however, the nuclear waste canister has a circular shell wall causing aprofile mismatch. Accordingly, some means is required to compensate fordimensional and profile differences at the peripheral fuel basket tocanister interface in order to center and stabilize the basket withinthe canister for handling and transport without damaging the fuelassemblies contained therein.

In one prior approach, the fuel basket is first positioned inside themetallic cylindrical canister and thereafter laterally centered thereinby multiple so-called “loose” extruded basket shims. FIG. 2B shows suchloose basket shims comprise multiple individual tubular extrusions ofvarious complex and compound cross-sectional shapes each forming anenclosed central cavity or space. The extrusions are inserted into themany differently configured lateral gaps or pockets formed between thestepped lateral exterior surfaces of the fuel basket and the cylindricalinner surface of the canister. The entire peripheral gap is typicallyfilled with the loose shims as shown, requiring many individual andcustom made cross-sectional shapes of shim tubes to achieve this asshown.

The prior use of unattached extruded loose shim tubes alone however hasseveral fabrication and performance drawbacks. First, the fuel basketrejects the decay heat emitted by the used or spent nuclear fuelassemblies to the outer canister shell via the loose basket shims whichserve as a heat conduction bridge. In theory, the heated canister shellin turn then emits the heat to the immediate surrounding environment tocool the canister and fuel assemblies. Without direct coupling of thetubular shims to fuel basket plates, however, thermal conduction is notas effective as desired.

The tubular shaped shim extrusions with solid walls further block astraight line of sight between the exterior surfaces of the fuel basketand the interior surface of the cylindrical canister. This blocks aradiant heat transfer path from the basket directly to the canister,which detrimentally reduces overall heat transfer effectiveness requiredto cool the fuel assemblies to prevent their structural degradation.Accordingly, because the heat rejection rate is limited by the looseextruded shims and the canister diameter is standardized to fit within aradiation shielded overpack or cask, a restriction is imposed on thesize of the fuel basket and concomitantly the number of fuel assemblieswhich can be stored therein to keep the assembles from overheating.

Secondly, as is further apparent in FIG. 2B, multiple sizes of shimswith various cross-sectional shapes must be fabricated to accommodateboth different diameter fuel baskets (i.e. lateral width dimensions inall lateral directions), and the many differently configuredcross-sectional shaped gaps formed between the basket periphery andcanister. Such a full set of shims of various dimensions and shapes fora single basket and canister assembly are obviously quite costly tomanufacture.

Finally, the “loose” extruded tubular shims do not structurallyreinforce or stiffen the fuel basket itself because there is no solidfixation of the shims to the slotted plate walls of the basket. When thebasket is therefore lifted and handled for insertion into the fuelcanister, the basket is susceptible to damage if banged against thecanister or outright dropped to the floor.

Accordingly, there remains a need for improvements in supporting,stabilizing, and centering fuel baskets in SNF canisters.

BRIEF SUMMARY

The present application discloses a nuclear fuel storage systemincluding a canister and fuel basket with integral fuel basket shimmingsystem. The present system is economical to manufacture and overcomesthe drawbacks of using the foregoing prior individual “loose” shimsalone to fill the many different size and shaped peripheral gaps formedbetween the fuel basket exterior side surfaces and the cylindrical fuelstorage canister. The present basket shimming system advantageouslyprovides a mechanical support system which directly reinforces the fuelbasket structurally separate from the canister to avoid damage wheninserting the basket therein, and laterally centers and stabilizes thebasket in the canister to resist movement and damage during seismicevents.

In addition, the present integral fuel basket shimming system improvesthe nuclear fuel assembly heat rejection rate by providing unobstructedstraight lines of sight between the lateral exterior side surfaces ofthe fuel basket and the canister shell for efficient radiant heattransfer. This direct radiant heat rejection path established betweenthe fuel basket and canister advantageously protects the structuralintegrity of the fuel assembles better. Moreover, larger capacity fuelbaskets holding a greater number of fuel assemblies can accordingly bestored in a single canister due to the improved heat rejection ratesobtainable.

In one implementation, a fuel basket incorporating the present shimmingsystem generally comprises a plurality of interlocked and orthogonallyintersecting slotted plates. The plates are built up in horizontal tiersor rows to the desired height of the fuel basket. At least some slottedplates comprise cantilevered lateral plate extensions on the ends whichprotrude perpendicularly beyond the flat lateral walls and exteriorperipheral surfaces of the adjoining plate on the sides of the basket.The vertical edge surfaces of the ends of the plate extensions areconfigured to terminate proximate to or abut the interior surface of thecylindrical canister. This restricts lateral movement of the fuel basketwithin the fuel storage canister in the case of a seismic event or ifdropped during handling such as insertion of the canister into aradiation shielded outer transfer or storage cask.

A plurality of differently configured lateral peripheral pockets,spaces, or gaps is formed between the exterior walls of the fuel basketand canister shell. In one embodiment, vertically elongatedreinforcement members are disposed in at least some of the gaps. Thereinforcement members are fixedly coupled directly to the fuel basket,and more specifically in some constructions to the slotted plateextensions that define part of peripheral gaps in conjunction with thecircular arcuate interior surfaces of the canister. The reinforcementmembers may have a height coextensive with the height of the fuelbasket. In certain embodiments, the reinforcement members may comprisereinforcement plates or a combination of reinforcement plates andtubular shim members both of which are fixedly coupled directly to thecantilevered basket plate extensions. This provides structuralreinforcement of the fuel basket plate extensions and in turn theoverall fuel basket structure in both the lateral direction and verticaldirection if subjected to compression forces in the event the basketwere dropped on its end during insertion into the canister, or if thecanister were similarly dropped after the basket is in place. Thestructural integrity of the fuel assemblies contained the fuel basket istherefore better protected overall.

The bottom edges of some or all of the lateral plate extensions andreinforcement members may further comprise flow cutouts or holes wherethe edges abuttingly engage the bottom closure plate of the canister.This allows the inert gas (e.g. helium or other inert gas) which fillsthe hermetically sealed cavity of the canister and protectively blanketsthe fuel assemblies therein to recirculate up and down via naturalconvective thermosiphon action driven by the gravity and the heatemitted from the decaying fuel assemblies. The peripheral spaces or gapsbetween the fuel basket and cylindrical shell of the canister act as agas flow downcomer of the gas recirculation circuit which is in fluidcommunication with the riser space formed inside the fuel basket throughthe nuclear fuel assemblies via the flow cutouts.

According to one aspect, a nuclear fuel storage system comprises: acanister comprising a cylindrical shell extending along a verticalcenterline; a fuel basket positioned in the canister, the fuel basketformed by a plurality of orthogonally arranged and interlocked slottedplates which collectively define exterior side surfaces of the fuelbasket; the fuel basket comprising a plurality of interior cells beingdefined by the slotted plates, each interior cell configured to hold afuel assembly comprising spent nuclear fuel; at least some of theslotted plates comprising cantilevered plate extensions, the plateextensions protruding laterally beyond the exterior side surfaces of thefuel basket and defining peripheral gaps between the fuel basket and thecanister; wherein the plate extensions are configured to engage theshell of the canister.

According to another aspect, a nuclear fuel storage system comprises: acanister comprising a cylindrical shell extending along a verticalcenterline; a fuel basket positioned in the canister, the fuel basketdefining a grid array of interior cells each of which is configured tohold a fuel assembly comprising spent nuclear fuel; the fuel basketcomprising a plurality cantilevered plate extensions, the plateextensions protruding laterally beyond vertical exterior side surfacesof the fuel basket and defining peripheral gaps between the fuel basketand the canister; and a plurality of vertically elongated reinforcementmembers positioned in the peripheral gaps, the reinforcement memberseach being fixedly coupled to the plate extensions.

According to another aspect, a method for forming a structurallyreinforced fuel basket for storing nuclear fuel comprises: providing afuel basket comprising a plurality of vertically stacked and interlockedslotted plates collectively defining a plurality of vertical exteriorsidewalls, a plurality of lateral plate extensions protruding laterallyoutward beyond the exterior sidewalls, and a plurality of storage cellseach of which is configured to hold a fuel assembly comprising spentnuclear fuel; fixedly coupling a plurality of vertically elongatedreinforcement members to the plate extensions; and inserting the fuelbasket with coupled reinforcement members into an upwardly cavity of acylindrical nuclear fuel storage canister. In some embodiment, thefixedly coupling step includes bolting the reinforcement members to thelateral plate extensions.

Further areas of applicability of the present invention will becomeapparent from the detailed description provided hereinafter. It shouldbe understood that the detailed description and specific examples, whileindicating the preferred embodiment of the invention, are intended forpurposes of illustration only and are not intended to limit the scope ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description and the accompanying drawings, wherein likeelements are labeled similarly and in which:

FIG. 1 is a perspective view of a prior nuclear fuel basket;

FIG. 2A is a front view of a prior slotted panel or plate design used inthe construction of the fuel basket of FIG. 1;

FIG. 2B is a top view of a prior spent nuclear fuel (SNF) canister withfuel basket of FIG. 1 and prior “loose” type shims;

FIG. 3 is a perspective view of an improved SNF canister according tothe present disclosure;

FIG. 4 is a side view of slotted plate designs according to the presentdisclosure showing examples of a top slotted plate, middle slottedplate, and bottom slotted plate used to construct a fuel basketaccording to the present disclosure;

FIG. 5A is a top perspective view of a first embodiment of a fuel basketconstructed with the slotted plates of FIG. 4 for use in the fuelcanister of FIG. 3, with some fuel basket reinforcement members beingdrawn out in exploded view;

FIG. 5B is an enlarged detail taken from FIG. 5A;

FIG. 6 is a top perspective view showing a first embodiment of areinforcement member assembly from FIG. 5A;

FIG. 7 is a top perspective view showing a second embodiment of areinforcement member assembly from FIG. 5A;

FIG. 8 is a top plan view of the canister and fuel basket of FIG. 5A;

FIG. 9A is a top perspective view of a second embodiment of a fuelbasket constructed with the slotted plates of FIG. 4 for use in the fuelcanister of FIG. 3, with some fuel basket reinforcement members beingdrawn out in exploded view;

FIG. 9B is an enlarged detail taken from FIG. 9A;

FIG. 10 is a top perspective view showing a third embodiment of areinforcement member assembly from FIG. 9A;

FIG. 11 is a top perspective view showing a fourth embodiment of areinforcement member assembly from FIG. 9A;

FIG. 12 is a top plan view of the canister and fuel basket of FIG. 9A;

FIG. 13A is a vertical cross-sectional view taken from FIG. 12;

FIG. 13B is a partial vertical cross sectional view of the canister andfuel basket of FIG. 13A showing portions of an inert gas recirculationcircuit and directional flow arrows thereof;

FIG. 14A is a top perspective view of a third embodiment of a fuelbasket constructed with the slotted plates of FIG. 4 for use in the fuelcanister of FIG. 3;

FIG. 14B is an enlarged detail taken from FIG. 14A;

FIG. 15A is a top perspective view of the fuel basket of FIG. 14Ashowing some fuel basket reinforcement members being drawn out inexploded view;

FIG. 15B is an enlarged detail taken from FIG. 15A;

FIG. 16 is a top perspective view showing a fifth embodiment of areinforcement member assembly from FIG. 14A;

FIG. 17 is a top perspective view showing a sixth embodiment of areinforcement member assembly from FIG. 14A;

FIG. 18 is a top perspective view showing a second embodiment of areinforcement member assembly from FIG. 5A;

FIG. 18 is a top plan view of the present canister and fuel basket ofFIG. 14A; and

FIG. 19 is a perspective view of a nuclear fuel assembly usable in thefuel baskets of FIGS. 5A, 9A, and 14A.

All drawings are schematic and not necessarily to scale. Features shownnumbered in certain figures which may appear un-numbered in otherfigures are the same features unless noted otherwise herein. A generalreference herein to a figure by a whole number which includes relatedfigures sharing the same whole number but with different alphabeticalsuffixes shall be construed as a reference to all of those figuresunless expressly noted otherwise.

DETAILED DESCRIPTION

The features and benefits of the invention are illustrated and describedherein by reference to non-limiting exemplary (“example”) embodiments.This description of exemplary embodiments is intended to be read inconnection with the accompanying drawings, which are to be consideredpart of the entire written description. Accordingly, the disclosureexpressly should not be limited to such exemplary embodimentsillustrating some possible non-limiting combination of features that mayexist alone or in other combinations of features.

In the description of embodiments disclosed herein, any reference todirection or orientation is merely intended for convenience ofdescription and is not intended in any way to limit the scope of thepresent invention. Relative terms such as “lower,” “upper,”“horizontal,” “vertical,”, “above,” “below,” “up,” “down,” “top” and“bottom” as well as derivatives thereof (e.g., “horizontally,”“downwardly,” “upwardly,” etc.) should be construed to refer to theorientation as then described or as shown in the drawing underdiscussion. These relative terms are for convenience of description onlyand do not require that the apparatus be constructed or operated in aparticular orientation. Terms such as “attached,” “affixed,”“connected,” “coupled,” “interconnected,” and similar refer to arelationship wherein structures are secured or attached to one anothereither directly or indirectly through intervening structures, as well asboth movable or rigid attachments or relationships, unless expresslydescribed otherwise.

As used throughout, any ranges disclosed herein are used as shorthandfor describing each and every value that is within the range. Any valuewithin the range can be selected as the terminus of the range. Inaddition, any references cited herein are hereby incorporated byreference in their entireties. In the event of a conflict in adefinition in the present disclosure and that of a cited reference, thepresent disclosure controls.

As used herein, the terms “seal weld or welding” if used herein shall beconstrued according to its conventional meaning in the art to be acontinuous weld which forms a gas-tight joint between the parts joinedby the weld.

FIGS. 3-8 depict a nuclear fuel canister 100 with a first embodiment ofa nuclear fuel basket 200 comprising of a hybrid integral shimmingsystem according to the present disclosure for centering, supporting,and reinforcing the basket structure. The shimming system utilizes aplurality of reinforcement members including a combination ofreinforcement plates 250 and tubular shim members 260 all fixedlycoupled to the fuel basket as further described in detail below. Thiscontrasts to the use of prior “loose” basket shims which are not affixedto the fuel basket and therefore do not structural reinforce the fuelbasket outside of the canister. The present reinforcement members arepositioned in some, but not necessarily all peripheral pockets or gaps Gformed between the fuel basket and the canister. The reinforcementplates 250 may generally have a smaller more compact cross-sectionalprofile allowing them to be used in tighter/smaller peripheral gaps,whereas the tubular shim members 260 with larger cross-sectionalprofiles can be used in the larger gaps.

Canister 100 may be used for storing any type of high level radioactivenuclear waste, including without limitation spent nuclear fuel (SNF) orother forms of radioactive waste materials removed form the reactor. TheSNF canister 100 may be any commercially-available nuclear fuel/wastestorage canister, such as a multi-purpose canister (MPC) available fromHoltec International of Camden, N.J. or other.

Canister 100 has a vertically elongated and metallic body a cylindricalshell 101 extending along a vertical centerline Vc which passes throughthe geometric center of the shell. Canister 100 includes a bottomclosure plate 102 seal welded to a bottom end of the shell, and a topclosure plate 103 seal welded to a top end of the shell. A hermeticallysealed cavity 104 is therefore formed inside the canister such as viaseal welding the closure plates to the shell ends. The foregoingcanister parts may be formed of any suitable metal, such as for examplewithout limitation steel including stainless steel for corrosionprotection.

Fuel basket 200 is a honeycomb prismatic structure which comprises a top230, bottom 231, and plurality of stepped and rectilinear shapedperipheral sides 232 defining exterior sidewalls 232A extendingvertically therebetween. The structure of the fuel basket 200 is formedby a plurality of the interlocked and orthogonally intersecting slottedplates 210. Slotted plates 210 are horizontally elongated in length eachhaving a length (measure transversely to vertical centerline Vc)substantially greater than their height (e.g. at least 4 times theheight or more). Peripheral sides 232 of the outermost exterior slottedplates 210 define outward facing exterior peripheral side surfaces 233which are collectively formed by the slotted plates 202. Plates 210 maybe continuous monolithic structures which extend diametrically/laterallyfrom one side of the canister 100 to the opposite side as shown. Theelongated slotted plates 210 each define a centerline longitudinal axisLA extending along the length of the plate.

In one embodiment, slotted plates 210 each include flat and parallelopposing vertical major sides surfaces 211, 212, a top longitudinal edge213, bottom longitudinal edge 214, and opposing ends 215 definingperipheral side edges 217 of the plates. To interlock the plates, aplurality of longitudinally spaced apart vertical slots 216 are formedperpendicularly to longitudinal axis LA in the top longitudinal edge,bottom longitudinal edge, or both of each (depending on the location ofthe plate in the fuel basket 200). The plates are orientedhorizontally/laterally and interlocked with each other via the slots 216to form a stacked structure comprised of multiple horizontal rows ortiers of plates rising and stacked to the desired height of the fuelbasket. The fuel basket may be slightly shorter than the canister 100 inheight such that an upper flow plenum 200A is formed between top 230 ofthe basket and the canister top closure plate 103 (see, e.g. FIGS.13A-B), which is further described herein. The uppermost plates 210Afrom the top tier comprise only downwardly open slots 216, the lowermostplates 210C from the bottom tier comprise only upwardly bottom slots216, and those intermediate plates 210B in middle tiers therebetweencomprise both top and bottom slots 216. As shown, the slots 216 extendonly partially through the entire height H1 of the slotted plates,approximately 50% or less of height H1 in some embodiments. The plates210A-C may have the same or different heights and lengths depending ontheir location with the fuel basket structure.

Fuel basket 200 includes a grid array of plural vertically-extendingfuel assembly storage cells 240 in its interior. Each cell is configuredin cross-sectional area and shape to hold a single U.S. style fuelassembly 28, which contains multitude of spent nuclear fuel rods 28 a(or other nuclear waste). An exemplary fuel assembly of this type havinga conventional rectilinear cross-sectional configuration is shown inFIG. 19. Such fuel assemblies are well known in the industry. The cells240 of the fuel basket are defined by the orthogonally intersectingslotted plates 210, and therefore a concomitantly rectilinearcross-sectional shape (e.g. square). This gives the fuel basket anoverall compound rectilinear polygonal shape in transverse cross sectionas shown which includes multi-faceted and stepped exterior peripheralside surfaces 233 collectively defined by the flat lateral peripheralsidewalls 232A of the outermost exterior slotted plates 210.

A plurality of peripheral spaces or gaps G are formed around theperimeter of the fuel basket 200 between the peripheral sidewalls 232Aand interior surface 110 of the fuel canister 100. The gaps G extendvertical for the full height of the fuel basket in the canister. Asshown for example in FIG. 8 et al., the gaps G vary in size andconfiguration, but generally have the same compound arcuate shape on theoutermost portion attributed to the cylindrical canister shell 101, anda rectilinear shape on the innermost portions attributed to the fuelbasket geometry which collectively define the gaps.

To laterally stabilize and center the fuel basket 200 in the canister100, and compensate for the mismatch between the rectilinear polygonalexterior peripheral side surfaces 233 of fuel basket 200 andnon-polygonal and circular arcuate interior surface 110 of canister 100,at least some of the slotted plates 200 comprise a cantilevered lateralplate extension 220 formed on one or preferably both ends. When theslotted plate are interlocked and assembled into the final fuel basketassembly, the lateral plate extensions 220 protrude perpendicularly andlaterally outward beyond the flat outwardly facing exterior peripheralside surfaces 233 of the adjoining plate which is orientedperpendicularly to the extension (see, e.g. FIGS. 5B and 8). It bearsnoting that the vertical end surfaces of the plate extensions 220 definethe plate ends 215 and concomitantly peripheral side edges 217 of theslotted plates 210 previously described herein. The peripheral sideedges of the extensions are configured to terminate proximate to or abutthe interior surface of the cylindrical shell 101 of canister 100. Thisensures contact between the slotted plates and canister shell tostabilize the fuel basket 200 in the lateral direction during theoccurrence of a seismic event which can shake the fuel canister 100 andnuclear fuel assemblies 28 therein. It bears noting that not all slottedplates 210 necessarily require plate extensions 220.

It further bears particular noting that the peripheral side edges 217defined by the plate extensions 220 of the present slotted plates 210shown in FIG. 4 each form a straight vertical linear peripheral edgewhich extends for the full height H1 of the slotted plate 210 from toplongitudinal edge 213 to bottom longitudinal edge 214. This contrasts tothe undulating edge formed in conventional slotted plate designs whichinclude a tab projection for insertion into a slot-shaped hole of aperpendicularly mating slotted plates (compare FIGS. 2A and 4). In suchprior designs shown in FIG. 2A, the tabs do not extend beyond theperpendicularly mating plate as shown in any manner sufficient to formany structural extensions of the plate sufficient to attach and supportany other elements in the peripheral gaps between the fuel basket andcylindrical canister shell.

The present slotted plates 210 may have a monolithic body of unitarystructure in one embodiment from end to end 215. Plates 210 may beformed of any suitable material. One non-limiting example is a metal,such as preferably a corrosion resistant metal like stainless steel. Forenhanced radiation blocking, some or all of the slotted plates mayalternatively be formed of suitable radiation shielding materials suchas a boron-containing material like Metamic® (a proprietary product ofHoltec International of Camden, N.J.). Metamic® is a discontinuouslyreinforced aluminum boron carbide metal matrix composite materialdesigned for neutron radiation shielding. Other material may be used forplates 210 in certain embodiments.

With continuing reference to FIGS. 3-8, the fuel basket 200 in anotheraspect further includes a plurality of vertically-extending andelongated reinforcement members or plates 250. Reinforcement plates 250are positioned in at least some of the peripheral gaps G in the canister100 between the exterior peripheral side surfaces 233 of the fuel basket200 and interior surface 110 of the fuel canister 100. The plates 250are preferably rigidly affixed and coupled to the fuel basket, such aswithout limitation slotted plate extensions 220 as shown. Thereinforcement plates 250 provide structural support and reinforcement ofthe extensions and fuel basket 200 in both (1) the vertical directionunder compression forces in the event the basket were dropped on its endduring insertion into the canister 100 (or if the canister were droppedon its end with the fuel basket installed), and (2) the lateraldirection. Because the plate extensions 220 are cantilevered withoutlateral support, they are more susceptible to bending and damage ifimpacted and not buttressed by the reinforcement plates depending on theunsupported length of the cantilever.

The reinforcement plates 250 may be provided in coordinated pairs ofplates of the same or different configurations in some embodiments. Atleast some of the slotted plate extensions 220, and a majority incertain instances may be fitted with mating pairs of reinforcementplates 250. In certain embodiments, a majority of slotted plateextensions 220 may be coupled to a pair of reinforcement plates 250(see, e.g. FIGS. 14-18). One reinforcement plate 250 in each pair may becoupled to opposite major side surfaces 211, 212 of the slotted plate210 on extensions 220 as shown in certain embodiments. These plateextensions 220 may therefore be sandwiched between pairs ofreinforcement plates 250 which structurally reinforce the plateextensions from both sides.

In certain embodiments of fuel baskets 200, a single reinforcement plate250 may be fixedly coupled to one side of some of the plate extensions220 at certain locations as needed around the peripherally of thebasket. Such an arrangement is shown in FIGS. 5-8 further describedbelow.

The reinforcement plates 250 may extend vertically for the full heightof the basket 200 from the top 230 to bottom 231 thereby having acoextensive height to the basket. Plates 250 may be formed of anysuitable metal, such as preferably a corrosion resistant metal such asfor example without limitation aluminum or stainless steel. Other metalsmay be used.

Reinforcement plates 250 may have a variety of polygonal transversecross-sectional shapes (e.g. rectilinear) as needed to match theconfiguration of the peripheral spaces or gaps G between the fuel basket200 and cylindrical canister shell 101 where they plates are to beinstalled. The plates 250 may have typical structural shapes used inindustry and are laterally open structures which do not define aninterior space (unlike the tubular shimming members 260 furtherdescribed herein). Non-limiting examples of some rectilinear shapeswhich may be used for reinforcement plates 250 include for examplewithout limitation straight reinforcement plates 250A having a flatstrap-like body and L-shaped angled reinforcement plates 250B having anangled body similar to a structural angle (see, e.g. FIG. 8). The angledreinforcement plates 250B may have perpendicularly oriented legs ofequal or unequal width as needed to match the cross-sectional shape andgeometry of the peripheral G in which the plates are to be positioned.Other polygonal shapes, non-polygonal shapes, or combinations thereofmay be used for reinforcement plates 250 as needed to match the shape ofthe mating associated peripheral gap G in which the plates are to bepositioned.

Single or pairs of reinforcement plates 250 may be fixedly coupled tothe lateral plate extensions 220 of slotted plates 210 via any suitablecoupling mechanism. In one embodiment, reinforcement plates 250 may bebolted to the slotted plate extensions via bolts 251 which compriseassemblies of the elongated threaded bolt body, nuts, and washers asshown. The nuts may be tack welded to the bolt bodies after assembly tothe fuel basket plate extensions 220 to prevent their loosening. Thebolt bodies are received through bolt holes 265 formed in the plateextensions 220 and reinforcement plates 250 at the bolting locations.The reinforcement plates 250 are bolted to at least the plate extensions220 of the uppermost and lowermost slotted plates 210 in the fuel basketassembly. Multiple bolts may be used at these upper and lower locationsto fixedly couple the reinforcement plate 250 to the slotted plateextensions 220 as shown. Preferably, the reinforcement plates may befurther bolted to some intermediate plate extensions 220 therebetweenalong the height of the fuel basket 220 in a vertically spaced apartmanner for added securement of the extensions to the fuel basket 200(see, e.g. FIG. 5A). The bolting may be completed to rigidly affix thereinforcement plates 250 to the fuel basket 200 before the basket 200 isslideably inserted into the cavity 104 of the fuel canister 100.

It will be appreciated that the reinforcement plates 250 are onlypositioned within the peripheral pockets or gaps G outboard of theperipheral sides 232 of the fuel basket 200 around its perimeter, andnot the interior. Because the bolting preferably does not protrude intoany of the fuel assembly storage cells 240 of the basket 200, the bolts251 do not interfere with sliding and loading the spent fuel assemblies28 into the cells.

Reinforcement plates 250 may be formed of a suitable metal such as hightemperature tolerant materials like Aluminum Alloy 2219 or other,corrosion resistant steel such as stainless steel, or other metal whichmay be extruded or otherwise formed to shape.

With continuing reference to FIGS. 3-8, the hybrid shimming system offuel basket 200 further includes reinforcement members in the form oftubular shim members 260 fixedly coupled to the basket (e.g. plateextensions 220). Shim members 260 may occupy peripheral gaps G which donot contain reinforcement plates 250 in some embodiments. This providesseveral advantages. For example, the tubular shim members may bepositioned into larger peripheral gaps G around the perimeter of thefuel basket to add greater structural stability and reinforcement of thebasket in those locations. The smaller peripheral gaps may bestructurally reinforced by use of the reinforcement plates 250 in thoselocations which can be fabricated and are shaped to fit such tighterspaces more readily. By integrating the tubular shim members 260 intothe basket structure by rigid fixation thereto, a greater resistance ofthe fuel basket to compressive forces acting on the ends of the shimmembers and basket is achieved since the shim tubes act as columns whichstructurally can withstand greater compressive forces if the canister(or fuel basket alone before insertion therein) were dropped duringhandling than the reinforcement plates 250 alone. The tubular shimmembers 260 also offer greater resistance to laterally directed forceson the fuel basket and canister in the event of a drop at leastpartially on the side of the basket or canister after basket insertiontherein. This advantageously offers better protection for the fuelassemblies in the basket from physical damage during a drop event. Inaddition, fabrication costs can be reduced for the fuel basket 200 sincethe number of tubular shim members 260 can be minimized when combinedwith the reinforcement plates 250 which generally have simplerrectilinear shapes (in cross section) as shown that are less costly tofabricate.

The tubular shim members 260 each comprise vertically elongated bodiesdefining a top end 261, bottom end 262, and sidewalls 263 extendingtherebetween which defines an enclosed central opening 264. Top andbottom ends 261, 262 may be open to the central opening 264. Tubularshim members 260 may have a height coextensive with the height of thefuel basket, and further with the height of the reinforcement plates250. Both the tubular shim members 260 and reinforcement plates 250 areoriented parallel to the vertical centerline Cv of the canister 100.

Tubular shim members 260 may have a variety of transversecross-sectional shapes as need to complement the geometry of theperipheral gaps G in which they are positioned. Shim members 260 maytherefore have a polygonal cross-sectional shape (e.g. rectangular,square, triangular, hexagonal, etc.), non-polygonal cross-sectionalshape (e.g. circular, etc.), or combinations thereof. In the illustratedembodiment, a combination of rectilinear tubular shim member 260Acomprised of four substantially straight rectangular walls, and partialsquare shim members 260B comprised of three perpendicularly orientedinner straight walls, and an outer arcuately curved wall extendingtherebetween are provided. Other shapes may be used depending on thecross-sectional shape of the peripheral gap G. For example, theembodiment of FIGS. 9-12 further described herein show tubular shimmembers 260C having a partial triangular cross-sectional shapecomprising two perpendicularly oriented inner walls and an outerarcuately curved wall extending therebetween which matches the curve ofthe canister 100 interior surface.

Similarly to reinforcement plates 250, tubular shim members 260 may befixedly coupled to the slotted plate extensions 220 of fuel basket 200in the same manner such as in one embodiment via bolts 251 previouslydescribed herein which comprise assemblies of the threaded bolt body,nuts, and washers. The shim members 260 are bolted to at least the plateextensions 220 of the uppermost and lowermost slotted plates 210 in thefuel basket assembly. The vertical central space 264 and open top andbottom ends 261, 262 of the tubes provide access to the boltingnecessary to tighten the fastener assemblies. Accordingly, part of thebolts 251 protrudes into the central spaces of the tubes as shown.Multiple bolts may be used at these upper and lower locations to fixedlycouple the reinforcement plate 250 to the slotted plate extensions 220.

In some embodiments as shown, the same bolts 251 may be used to fixedlycouple both a reinforcement plate 250 and a tubular shim member 260 to asingle slotted plate extension 220. The plate extension may therefore besandwiched in an assemblage between the reinforcement plate and shimtube (best shown in FIG. 8). In some instances, a pair of tubular shimmembers 260 may occupy the same peripheral pocket or gap G (see, e.g.FIG. 8, top right detail image).

In other less preferred but still satisfactory embodiments, the tubularshim members 260 and/or reinforcement plates 250 may be welded to theslotted plate extensions 220. Bolting shim members 260 and reinforcementplates 250 to the fuel basket plate extensions 220 obviates any issueswith forming dissimilar metal welds and offers fabrication savings sincebolting is generally a less expensive coupling procedure than welding.

Tubular shim members 260 may be formed of a suitable metal such as hightemperature tolerant materials like Aluminum Alloy 2219 or other,corrosion resistant steel such as stainless steel, or other metal whichmay be extruded or otherwise formed to shape.

With continuing reference to FIGS. 4-8, the bottom edges of some or allof the lateral plate extensions 220, reinforcement plates 250, and/ortubular shim members 260 may further comprise flow holes or cutouts 270where the edges abuttingly engage the bottom closure plate 102 of thecanister 100. This provides flow access to the fuel assembly storagecells 240 in the basket 200 which allows the inert gas (e.g. helium orother inert gas) circulating and contained in cavity 104 of the canisterto blanket the fuel assembles for corrosion protection. The flow cutouts270 allow the gas to recirculate up and down within the canister vianatural convective thermosiphon action driven by the heat emitted fromthe decaying fuel assemblies 28. The peripheral spaces or gaps G betweenthe fuel basket and cylindrical shell 101 of the canister 100 act as agas flow downcomer of the gas recirculation circuit which is in fluidcommunication via cutouts 270 with the interior space formed by thecells 240 inside the fuel basket which contain the fuel assemblies. Theupper flow plenum 200A formed in the canister above the top of the fuelbasket 200 is in fluid communication with both the peripheral downcomerand interior riser. This is best shown in FIG. 13B which depicts inertgas flow arrows showing the gas flow recirculation circuit used to coolthe fuel assembly.

FIGS. 9-13B depict a nuclear fuel canister 100 with fuel basket 200comprising a second embodiment of a hybrid integral shimming systemaccording to the present disclosure which structurally reinforces andcenters/stabilizes the fuel basket 200 in the canister. This embodimentis similar to the first embodiment shown in FIGS. 3-8 in that itcombines reinforcement members comprising both reinforcement plates 250and tubular shim members 260; albeit some of slightly differentconfiguration. These reinforcement members are fixedly bolted to thefuel basket cantilevered lateral plate extensions 220 and have avertical height coextensive with the full height of the fuel basket.These reinforcement members may include flow cutouts 270 (also formed inthe slotted plate extensions 220 as previously described herein. Allsimilar features of the canister, fuel basket, and shimming system willtherefore not be repeated here in detail for the sake of brevity.Differences will be described with a general overview of the secondembodiment as well.

Although the present embodiment (and the prior embodiment in FIGS. 5-8)uses a combination of reinforcement members comprised of bothreinforcement plates 250 and tubular shim members 260, it bears notingthat in other embodiments only reinforcement plates or only tubular shimmembers may be used depending on the configuration/geometry and sizes ofthe peripheral gaps G formed between the canister shell 101 and exteriorsidewalls 232A of the fuel basket.

Referring to FIGS. 9-13B, fuel basket 200 comprises more interior fuelassembly storage cells 240 (e.g. 44 cells as shown) than the number ofcells in the embodiment of the fuel basket of FIGS. 3-8 (e.g. 37 cellsas shown). The diameter of the canister 100 however may be same in bothembodiments in certain instances. The 37 cell fuel basket represents thelargest capacity previously available on the market, which is limited atleast in part by the lower heat rejection capacity of prior basketdesigns which dissipates heat emitted by the decaying nuclear fuelassembly. Accordingly, the greater heat rejection capability of thepresent fuel basket designs using the integral shimming systemadvantageously allows for a greater number of fuel assemblies to besafely stored in the canister without increasing the diameter.

The reinforcement members in the present embodiment of FIGS. 9-13Binclude perpendicularly angled reinforcement plates 250B previouslydescribed herein. In this embodiment of the fuel basket shimming system,angled plates 250B have legs of unequal length to fit the peripheral gapG in which they are located. The tubular shim members include the foursided shim members 260B and generally triangular shaped shim members260C previously referred to. In this embodiment, shim members 260B and260C are fixedly coupled together via bolts 251 which passlaterally/horizontal therethrough and through lateral plate extensions220 of the fuel basket which is sandwiched between the shim members andextension as shown. The shim members 260B and 260C occupy differentperipheral gaps G on each side of the plate extension.

FIGS. 14-18 depict a nuclear fuel canister 100 with fuel basket 200comprising a third embodiment of a hybrid integral shimming systemaccording to the present disclosure which structurally reinforces andcenters/stabilizes the fuel basket 200 in the canister. Fuel basket 200in this embodiment comprises more interior fuel assembly storage cells240 (e.g. 104 cells as shown) than the number of cells in either thefuel baskets of FIG. 3-8 or 9-13B. The canister diameter however may bethe same as in these prior embodiments described.

The increased number of storage cells 240 results in peripheral gaps Ggenerally too small to practically accommodate tubular shimming members260 having larger transverse cross-sectional profile. This embodimentincludes reinforcement members comprising only reinforcement plates 250Aof the straight and flat strap-shaped design previously describedherein. In some peripheral gaps G, a pair of perpendicularly orientedreinforcement plates 250A may be positioned in a single gap and fixedlycoupled to different orthogonally oriented plate extensions 220 as shownto collectively form an L-shaped angled support. These 2-pieceassemblies of reinforcement plates 250A may be replaced in someembodiments by a single monolithic angled reinforcement plate 250Bpreviously described herein. The reinforcement plates are fixedly boltedto the fuel basket cantilevered lateral plate extensions 220 by bolts251 and have a vertical height coextensive with the full height of thefuel basket. These reinforcement members may also include flow cutouts270 as needed (also formed in the slotted plate extensions 220 aspreviously described herein).

All other similar features of the canister, fuel basket, and presentshimming system as the prior shimming system embodiments described willtherefore again not be repeated here in detail for the sake of brevity.

A method for forming a structurally reinforced fuel basket for storingnuclear fuel will now be briefly described. The steps of the method mayinclude interlocking a plurality of slotted plates 210 to form a fuelbasket comprising a plurality of vertical exterior sidewalls 232A and arectilinear grid array of a plurality of fuel assembly storage cells240. The interlocked slotted plates 210 are configured such that thecantilevered lateral extensions 220 are formed which protrude outwardsbeyond the sidewalls 232A along the entire height of the fuel basket.

With the fuel basket 200 thus provided, the method continues withfixedly coupling a plurality of the vertically elongated reinforcementmembers directly to the lateral plate extensions 220 of the fuel basket.The reinforcement members may include reinforcement plates 250 and/ortubular shimming members 260. In one embodiment, the plates and shimmingmembers are bolted to the lateral plate extensions 220 using bolts 250previously described herein and extend for the full height of the fuelbasket 200. The method may further include forming a gas flow pathbetween the peripheral gaps G at the bottom of the fuel basket 200 andthe interior fuel storage cells 240 by providing the flow cutouts 270 inthe bottom end of some or all of the reinforcement plates 250 if used,tubular shimming members 260 if used, and the lateral plate extensions220 of the lowermost tier of slotted plates 210 abutting the bottomclosure plate 102. The reinforcement members structurally reinforce thefuel basket to protect the integrity of the fuel storage cells and fuelassemblies to be stored therein as previously described.

The reinforced fuel basket 200 is next inserted and slid through theopen top end of the canister 100 into the cavity 104. The bottom closurelid 102 has already been hermetically seal welded onto the bottom end ofthe canister previously. The fuel basket becomes seated on the upwardfacing top surface of the bottom closure lid. One or more fuelassemblies 28 are then inserted into the storage cells 240 of thebasket. The top closure lid 103 is then hermetically seal welded ontothe top end of the canister 100 which completes the encapsulation of thefuel basket and fuel assembly (see, e.g. FIGS. 13A-B. The canister 100may then be filled with an inert gas (e.g. helium, nitrogen, etc.) whichbegins to flow in the gas recirculation circuit within the canisterdriven by the natural convective thermo-siphon as the gas is heated bythe fuel assemblies 28.

Numerous advantages can be realized with the present fuel basketintegral shimming system embodiments disclosed herein, which can besummarized as including but not limited to the following.

The bottom edges of the slotted plates 210, lateral plate extensions220, reinforcement plates 250 if provided, and tubular shim members 260if provided may include flow cutouts 270 as previously described hereinto enable inert gas (e.g. helium or other) recirculation by the naturalgravity-driven thermosiphon action. It bears particular note that notall of these components require flow cutouts depending on their locationand orientation within the fuel canister 100 in relation to theperipheral gaps G which collectively form the downcomer of the inert gasrecirculation system within the canister.

The cross sectional area of the open peripheral spaces or gaps G aroundthe perimeter of the fuel basket 200, which serve as the downcomer forthe recirculating gas, is maximized by limited use of tubular shimsfixedly attached to the slotted plate extensions 220 which helps boostthe convective heat transfer to the canister shell wall and cooling ofthe fuel assemblies in the basket.

Each slotted plate extension 220 serves as a heat transfer extendedsurface (“fin”) projecting laterally outwards from the prismatic fuelbasket 200 to enhance dissipation of heat emitted by the fuel assembliesin the basket to the peripheral gaps and in turn to the canister shell101. These heat transfer fins or extended surfaces aid in helping todraw the waste heat from the interior of the basket and transferring theheat to the recirculating inert gas flowing downwards through in thedowncomer space defined by the peripheral gaps G internal to thecanister 100.

The flow holes or cutouts 270 provide a direct line of sight between thebottom of the stored fuel and the bottom region of the fuel storagecanister shell 100 which helps maximize radiative heating of the bottomregion of the canister fuel confinement boundary and thus alleviate therisk of stress corrosion cracking of the canister which can occur underthe right environmental conditions; a well known phenomenon and failuremechanism in the art.

The need to weld the exterior peripheral basket slotted panels or plates210 together is eliminated by bolting the reinforcement members (e.g.reinforcement plates 250 and/or tubular shimming members 260) to thefuel basket lateral plate extensions 220, thereby enabling the assembledbasket to be entirely weld-free and thus of high dimensional fidelity.This significantly reduces fabrication times and costs. Thereinforcement members structurally ties the upper and lowest slottedplates 210 together to lock the entire vertical stack of plates into astructurally stable assemblage.

The reinforcement plates 250 and tubular shim members 260, which extendall the down to the bottom edge of the fuel basket 200, help tostrengthen and stiffen the basket against axial inertial loads duringany postulated vertical drop event as well as against any relatedlaterally-acting inertial radial loads (if basket dropped partially onits side). During such a vertical drop event, the axial impact forcesare transmitted between the bottom closure lid of the canister 100 tothe opposite end top closure lid via the vertically-extendingreinforcement plates and shim members.

While the foregoing description and drawings represent some examplesystems, it will be understood that various additions, modifications andsubstitutions may be made therein without departing from the spirit andscope and range of equivalents of the accompanying claims. Inparticular, it will be clear to those skilled in the art that thepresent invention may be embodied in other forms, structures,arrangements, proportions, sizes, and with other elements, materials,and components, without departing from the spirit or essentialcharacteristics thereof. In addition, numerous variations in themethods/processes described herein may be made. One skilled in the artwill further appreciate that the invention may be used with manymodifications of structure, arrangement, proportions, sizes, materials,and components and otherwise, used in the practice of the invention,which are particularly adapted to specific environments and operativerequirements without departing from the principles of the presentinvention. The presently disclosed embodiments are therefore to beconsidered in all respects as illustrative and not restrictive, thescope of the invention being defined by the appended claims andequivalents thereof, and not limited to the foregoing description orembodiments. Rather, the appended claims should be construed broadly, toinclude other variants and embodiments of the invention, which may bemade by those skilled in the art without departing from the scope andrange of equivalents of the invention.

1. A nuclear fuel storage system comprising: a canister comprising acylindrical shell extending along a vertical centerline; a fuel basketpositioned in the canister, the fuel basket formed by a plurality oforthogonally arranged and interlocked slotted plates which collectivelydefine exterior side surfaces of the fuel basket; the fuel basketcomprising a plurality of interior cells being defined by the slottedplates, each interior cell configured to hold a fuel assembly comprisingspent nuclear fuel; at least some of the slotted plates comprisingcantilevered plate extensions, the plate extensions protruding laterallybeyond the exterior side surfaces of the fuel basket and definingperipheral gaps between the fuel basket and the canister; wherein theplate extensions are configured to engage the shell of the canister. 2.The system according to claim 1, wherein the lateral plate extensionsdefine vertical edges which terminate proximate to an interior surfaceof the canister.
 3. The system according to claim 2, wherein thevertical edges are linearly straight from top to bottom.
 4. The systemaccording to claim 1, further comprising a plurality of verticallyelongated reinforcement members inserted in the peripheral gaps formedbetween the exterior side surfaces of the fuel basket and the shell ofthe canister, the reinforcement members fixedly coupled to the fuelbasket.
 5. The system according to claim 4, wherein the reinforcementmembers are fixedly coupled to the plate extensions at the upper end ofthe fuel basket, the lower end of the fuel basket, and in between. 6.The system according to claim 5, wherein the reinforcement members havea height which is coextensive with a full height of the fuel basket. 7.The system according to claim 6, wherein the reinforcement memberscomprise vertically elongated reinforcement plates extending for a fullheight of the fuel basket.
 8. The system according to claim 7, whereinthe reinforcement plates being arranged in pairs on the plate extensionsof the fuel basket, the plate extensions being sandwiched between thepairs of the reinforcement plates.
 9. The system according to claim 8,wherein the pairs of reinforcement plates are bolted to the plateextensions by bolts which pass through the reinforcement plates andplate extensions.
 10. The system according to claim 9, wherein each pairof the reinforcement plates is bolted to a respective plate extensionproximate to a top end of the fuel basket by a plurality of the boltsand proximate to a bottom end of the fuel basket by a plurality ofbolts.
 11. The system according to claim 7, wherein the reinforcementplates have a straight strap-like shape.
 12. The system according toclaim 7, wherein the reinforcement plates have an angled shape.
 13. Thesystem according to claim 1, wherein the reinforcement members eachfurther comprise a flow cutout formed in a bottom end of thereinforcement members.
 14. The system according to claim 1, wherein theplate extensions of the fuel basket each further comprise a flow cutoutformed in a bottom end of the plate extensions.
 15. The system accordingto claim 14, wherein the peripheral gaps form a flow downcomer and theinterior cells form a riser which is in fluid communication via the flowcutouts to form a natural convective thermos-siphon flow recirculationcircuit.
 16. The system according to claim 15, further comprising aninert gas contained in the canister which circulates through the flowrecirculation circuit.
 17. The system according to claim
 7. wherein thereinforcement members further comprise a plurality of verticallyelongated tubular shim members positioned in some of the peripheralgaps, the tubular shim members each being fixedly coupled to arespective plate extension of the fuel basket.
 18. The system accordingto claim 17, wherein the tubular shim members are bolted to therespective plate extensions.
 19. The system according to claim 18,wherein the tubular shim members are bolted to the plate extensions bybolts which pass through the tubular shim members, plate extensions, andone of the reinforcement plates.
 20. The system according to claim 18,wherein the tubular shim members are positioned in the peripheral gapswhich do not contain a reinforcement plate.
 21. A nuclear fuel storagesystem comprising: a canister comprising a cylindrical shell extendingalong a vertical centerline; a fuel basket positioned in the canister,the fuel basket defining a grid array of interior cells each of which isconfigured to hold a fuel assembly comprising spent nuclear fuel; thefuel basket comprising a plurality cantilevered plate extensions, theplate extensions protruding laterally beyond vertical exterior sidesurfaces of the fuel basket and defining peripheral gaps between thefuel basket and the canister; and a plurality of vertically elongatedreinforcement members positioned in the peripheral gaps, thereinforcement members each being fixedly coupled to the plateextensions.
 22. The system according to claim 21, wherein thereinforcement members comprise elongated reinforcement plates ofstraight or angular configuration extending for a full height of thefuel basket, each reinforcement member being fixedly coupled to arespective plate extension in one of the peripheral gaps.
 23. The systemaccording to claim 22, wherein the respective plate extensions aresandwiched between a pair of the reinforcement plates.
 24. The systemaccording to claim 22, wherein the reinforcement members furthercomprise a plurality of vertically elongated tubular shim memberspositioned in some of the peripheral gaps, the tubular shim members eachbeing fixedly coupled to one side of a respective plate extensionopposite to one of the reinforcement plates fixedly coupled to anopposite side of the respective plate extension. 25-30. (canceled)